1. Field of the Invention
The present invention relates to a cylindrical electric double-layer capacitor.
2. Description of the Related Art
There is a conventionally known cylindrical electric double-layer capacitor including an electrode winding which is formed by superposing a band-shaped positive electrode and a band-shaped negative electrode one on another with a first separator interposed therebetween, superposing a second separator onto one of said band-shaped positive and negative electrodes to provide a superposed material and by spirally winding the superposed material such that the second separator is located on an outermost side, and a container having the electrode winding accommodated therein. In this case, in the electrode winding, that portion of one of the band-shaped positive and negative electrodes, which is located on an outermost periphery, namely, a last-lap annular outer peripheral surface, has no mating electrode opposed thereto.
Therefore, the conventionally known capacitor suffers from a disadvantage that the last-lap annular peripheral surface does not participate in the ensuring of the electrostatic capacity and hence, is useless and a difference between amounts of electrodes utilized for forming the band-shaped positive and negative electrodes causes an unbalance in potential to deteriorate the performance.
There is also a conventionally known cylindrical electric double-layer capacitor including an electrode winding which is formed by interposing one separator between band-shaped positive and negative electrodes expanded by the charging, superposing another separator onto one of said band-shaped positive and negative electrodes to provide a superposed material and by spirally winding the superposed material such that the another separator is located on an outermost side, and a container having the electrode winding accommodated therein. In this case, each of the band-shaped positive and negative electrodes is formed at a uniform thickness over the whole thereof using activated carbon as a predominant component. Examples of activated carbon expanded by the charging are alkali-activated carbon made from meso-phase pitch as a starting material.
The alkali-activated carbon has a high density and a high capacity and is extremely effective for enhancing the electrostatic capacity of a cylindrical electric double-layer capacitor. On the other hand, however, there is a possibility that the deformation or the like of the band-shaped positive and negative electrodes may occur to deteriorate the performance of the cylindrical electric double-layer capacitor. because the band-shaped positive and negative electrodes are expanded by the charging, as described above. Incidentally, such expansion advances in a given amount depending on the charged voltage, but little advances in a constant voltage charged state. The amounts of band-shaped positive and negative electrodes shrunk are smaller than the amounts of band-shaped positive and negative electrodes expanded.
Accordingly, it is an object of the present invention to provide a cylindrical electric double-layer capacitor of the above-described type, wherein entire opposite surfaces of the band-shaped positive and negative electrodes of the electrode winding are utilized effectively for ensuring an electrostatic capacity, and the deterioration in the performance can be inhibited to the utmost by the equalization of the amounts of electrodes utilized for the band-shaped positive and negative electrodes.
To achieve the above object, according to the present invention, there is provided a cylindrical electric double-layer capacitor comprising an electrode winding which is formed by superposing band-shaped positive and negative electrodes one on another with a first separator interposed therebetween, superposing a second separator onto one of the band-shaped positive and negative electrodes to provide a superposed material, and by spirally winding the superposed material such that the second separator is located on an outermost side, and a container having the electrode winding accommodated therein, wherein a cylindrical electrode is provided on an inner peripheral surface of the container opposed to an outer peripheral surface of the electrode winding, the polarity of the cylindrical electrode being set at a polarity opposite from the polarity of that portion of the one of the band-shaped positive and negative electrodes, which is located on an outermost periphery of the electrode winding.
If the cylindrical electric double-layer capacitor is constructed as described above, the last-lap annular outer peripheral surface which is a portion of one of the band-shaped positive and negative electrodes of the electrode winding has a mating electrode in opposed relation to the cylindrical electrode of the container having a polarity different from that of the last-lap annular outer peripheral surface with the second separator interposed therebetween. Thus, the entire opposite surfaces of the band-shaped positive and negative electrodes of the electrode winding can be utilized effectively for ensuring the electrostatic capacity, thereby increasing the electrostatic capacity to the value more than that in the conventionally known capacitor. In addition, the deterioration in the performance of the electric double-layer capacitor can be inhibited to the utmost by the equalization of the amounts of electrodes utilized for the band-shaped positive and negative electrodes.
It is another object of the present invention to provide a cylindrical electric double-layer capacitor of the above-described type, wherein the absolute amounts of band-shaped positive and negative electrodes expanded can be decreased to the values less than that in the conventionally known capacitor, and the absolute values of the weights of the band-shaped positive and negative electrodes can be maintained approximately equivalent to those in the conventionally known capacitor, whereby the deterioration in the performance can be inhibited to the utmost.
To achieve the above object, according to the present invention, there is provided a cylindrical electric double-layer capacitor comprising an electrode winding which is formed by interposing one separator between band-shaped positive and negative electrodes expanded by the charging, superposing another separator onto one of the band-shaped positive and negative electrodes to provide a superposed material, and by spirally winding the superposed material such that the other separator is located on an outermost side, and a container having the electrode winding accommodated therein, wherein the thickness of each of the positive and negative electrodes is decreased from a winding start point toward a winding end point.
In the electrode winding, the rate of increase in final thickness caused by the expansion of the band-shaped positive and negative electrodes is, for example, about 10% at the center portion, about 30% at an intermediate portion, and about 50% at an outer peripheral portion. The reason why the rate of increase in thickness is smaller at the center portion is that a clamping force provided by the winding is applied strongly to the center portion. On the other hand, the reason why the rate of increase in thickness is larger at the outer peripheral portion is that the clamping force applied to the outer peripheral portion is weak due to that a gap for fitting the electrode winding exists between the electrode winding and the container.
Therefore, when the band-shaped positive and negative electrodes are constructed as described above, the outer peripheral portion expanded in the largest amount can be formed at the smallest thickness to reduce the absolute amount of entire electrode winding expanded, as compared with the conventionally known capacitor, thereby avoiding the deformation or the like of the band-shaped positive and negative electrodes. On the other hand, the center portion expanded in the smallest amount can be formed at the largest thickness, whereby the absolute values of the weights of the band-shaped positive and negative electrodes can be maintained approximately equivalent to those in the conventionally known capacitor.
Thus, it is possible to inhibit the deterioration in the performance to the utmost, such as a decrease in electrostatic capacity, an increase in internal resistance and the like due to the expansion of the band-shaped positive and negative electrodes.
The above and other objects, features and advantages of the invention will become apparent from the following description of the preferred embodiment taken in conjunction with the accompanying drawings.